This invention relates to a method for coating a carrier with a coating suspension. More particularly, this invention relates to a method and an apparatus for coating carriers for catalysts, for example automotive exhaust-gas catalysts.
As a rule, the carriers for automotive exhaust-gas catalysts are of a cylindrical shape with two faces and a shell surface, and a large number of flow ducts for the exhaust gases of the internal combustion engines extend from the first face to the second face essentially in parallel to the cylinder axis. These carriers are also referred to as honeycomb carriers.
The cross-sectional shape of the carriers depends on the installation requirements in the motor vehicle. Carriers having a round cross-section, an elliptical or triangular cross-section are widely used. The flow ducts mostly include a square cross-section and are arranged in a closely packed grid pattern over the entire cross-section of the carriers. Depending on the application, the duct or cell density of the flow ducts varies between 10 and 140 cm−2. Honeycomb carriers having cell densities of up to 250 cm−2 are being developed.
For purifying automotive exhaust gases, catalyst carriers obtained by extruding ceramic bodies are mainly used. Alternatively, catalyst carriers made of corrugated and wound metal foils are available. For purifying passenger car exhaust gases, ceramic carriers having cell densities of 62 cm−2 are still being used predominantly. The cross-sectional dimensions of the flow ducts are 1.27×1.27 mm2 in this case. Wall thicknesses of such carriers range between 0.1 and 0.2 mm.
In order to convert the pollutants contained in automotive exhaust gases, such as carbon monoxide, hydrocarbons and nitrogen oxides, into harmless compounds, very finely divided metals of the platinum group are typically used, the catalytic effect of which may be altered by compounds of non-noble metals. These catalytically active components must be deposited onto the carriers. However, it is impossible to guarantee the required very fine dispersion of the catalytically active components by depositing these components onto the geometrical surfaces of the carriers. This applies equally to the non-porous metallic and porous ceramic carriers. A sufficiently large surface for the catalytically active components may be provided only by applying a support layer of finely divided (i.e. in powder form), high-surface area materials onto the internal surfaces of the flow ducts. In the following, this operation is referred to as coating of the carrier. Coating the shell surface of the carriers is not desired and should be avoided in order to prevent loss of valuable catalytically active materials.
A suspension of the finely divided, high-surface area materials in a liquid phase, normally water, is used for coating the carriers. As high-surface area support materials for the catalytically active components, typical coating suspensions for catalytic applications include, for example, aluminum oxides, aluminum silicates, zeolites, silicon dioxide, titanium oxide, zirconium oxide and oxygen-storing components on the basis of cerium oxide. These materials constitute the solids content of the coating suspension. In addition, soluble precursors of promoters or catalytically active noble metals of the platinum group in the periodic table may also be added to the coating suspension. The solids concentration of typical coating suspensions ranges between 20 and 65 wt.-% based on the total weight of the suspension. They exhibit densities between 1.1 and 1.8 kg/l.
According to the prior art, various methods for depositing the support layer onto the carriers using the coating suspension or slurry are known. In order to coat the carriers, they may be dipped into the coating suspension or coated by pouring the coating suspension over them. It is also possible to pump or suck the coating suspension into the ducts of the carriers.
In any case, surplus coating material must be removed from the ducts of the carriers by suction or by blowing-off with compressed air. This will also open ducts which may have become blocked by coating suspension.
After coating, the carrier and the support layer are dried and then calcined in order to solidify the support layer and fix it to the carrier. Subsequently, the catalytically active components are introduced into the coating by impregnation, using mostly aqueous solutions of precursor compounds of the catalytically active components. As an alternative, the catalytically active components may already be added to the coating suspension itself. In this case, subsequent impregnation of the completed support layer with the catalytically active components may be omitted.
An essential criterion of the coating methods is the coating or loading concentration which can be achieved in a single run using these methods. This signifies the solids content left on the carrier following drying and calcination. The coating concentration is indicated in grams per liter of volume of the carriers (g/l). In practice, coating concentrations of up to 300 g/l are needed for automotive exhaust-gas catalysts. If the method used is incapable of applying this quantity in a single run, the coating operation, following drying and, if necessary, calcination of the carrier, must be repeated until the desired loading is achieved. Frequently, two or more coating operations using coating suspensions of different compositions are performed. As a result, catalysts are obtained which include several layers stacked on top of each other and having different catalytic functions.
DE 40 40 150 C2 describes a method in which catalyst carriers having a honeycomb shape may be coated uniformly with a support layer and a catalytically active layer, respectively, over their entire lengths. Below, catalyst carriers will also be referred to as honeycomb carriers. According to the method described in DE 40 40 150 C2, the cylinder axis of the honeycomb carrier is aligned vertically for coating. Then, the coating suspension is pumped into the ducts through the lower face of the honeycomb carrier until it emerges at the upper face. After that, the coating suspension is pumped down again, and surplus coating suspension is blown or sucked out of the ducts in order to prevent the ducts from becoming blocked. This method produces support layers which exhibit good uniformity over the entire length of the honeycomb carriers.
The coating method described above includes a certain variation in coating quantities from one carrier to the other. This variation depends on the nature of the coating suspension and on the characteristics of the honeycomb carriers to be coated, such as their porosity, for example.